A number of techniques have been developed to reduce the amount of nitrogen oxides (NO.sub.x) generated from combustion processes. They include techniques that employ (a) post-combustion clean-up strategies such as selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR); (b) modification of operating conditions, for example adjustments in air/fuel ratios; (c) modifications of burner internals, with air staging and fuel staging illustrating this type of approach; and (d) modification of the combustion system, in particular, through the use of flue gas recirculation systems.
The majority of these techniques have been developed in relation to air-based combustion processes. As combustion systems that use oxygen or oxygen-enriched air are being adopted by a number of industries, NO.sub.x reduction techniques must address those aspects in the generation of NO.sub.x that are specific to oxygen-based combustion processes.
It is known that the amount of nitric oxides (NO.sub.x) produced in combustion increases with temperature and with the oxygen and nitrogen concentrations. When air is replaced with pure, nearly pure or with oxygen-enriched air, two competing effects are observed. On one hand, the amount of nitrogen available to form NO.sub.x is lowered; on the other, the flame temperature is increased. It must be noted, however, that even when the combustion is carried out using technically pure oxygen (oxygen concentration of 99.5% or higher), nitrogen levels in the furnace can be significant due to air infiltration through leaks in the furnace, or to using both oxygen-fired and air-fired burners.
One advance in lowering NO.sub.x levels generated during oxygen-based combustion processes involves mixing the oxidant stream with species present in the furnace prior to contact with the fuel stream. Although NO.sub.x reductions obtained by this technique are excellent, federal and state regulations are placing increasingly stringent targets for NO.sub.x emissions. Accordingly, there continues to be a need for combustion processes which generate lower and lower NO.sub.x levels and are still capable of effectively heating industrial furnaces such as might be used in glass or steel making.
It is an object of this invention to provide a process for manipulating the combustion system in order to reduce the generation of NO.sub.x.